1. Field of the Invention
The present invention relates to a creasing device that preliminary produces a fold mark or a crease in a sheet member (hereinafter, “sheet”) delivered from a preceding stage before the sheet is folded and to an image forming system that includes the creasing device and an image forming apparatus.
2. Description of the Related Art
What is called saddle-stitch or center-folded booklet production has been conventionally performed by saddle stitching a sheet batch, which is a stack of a plurality of sheets delivered from an image forming apparatus, and folding the thus-saddle-stitched sheet batch in the middle of the sheet batch. Folding such a sheet batch containing a plurality of sheets causes outside sheets of the sheet batch to be stretched at a fold line by a greater amount than inside sheets. An image portion formed at the fold line on outside sheets can thus be stretched, resulting in damage, such as come off of toner, to the image portion in some cases. A similar phenomenon can occur when other fold, such as z-fold or tri-fold, is performed. A sheet batch can be folded insufficiently depending on the thickness of the sheet batch.
A creasing device called a creaser that produces a fold mark (a crease) in a sheet batch prior to a folding process where the sheet batch undergoes half fold or the like to make outside sheets easy to fold, thereby preventing come off of toner have already been known. Some types of such creasing devices produce a crease in a sheet in a direction perpendicular to a sheet conveying direction by moving a roller on the sheet, burning the sheet with a laser beam, pressing a creasing blade against the sheet, or a like method.
However, producing the crease in a sheet with the roller involves moving the roller across a full length of the sheet in a direction, along which a fold extends, and therefore is time consuming. This can be resolved by rotating the sheet conveying direction by 90 degrees and producing a crease parallel to the sheet conveying direction; however, this scheme involves an effect on footprint and therefore is disadvantageous in view of space-saving. Creasing by using a laser beam is environmentally less favorable because smoke and odor are given off during creasing.
Creasing a sheet by pressing a creasing blade against the sheet can be performed in a relatively short period of time and allows easy production of a crease perpendicular to a sheet conveying direction; however, pressing a longitudinal face of the creasing blade against the sheet entirely at once causes a high load. To reduce the load, a scheme of bringing the creasing blade face into contact with a sheet in multiple batches can be used. However, this scheme is disadvantageous in that unevenness can develop between a portion that contacts the blade multiple times and a portion that contacts the blade only once and also in that producing a crease by making contact in multiple batches can decrease productivity.
To solve the inconveniences discussed above, it is possible to reduce a load placed on a creasing moving unit by bringing a creasing blade gradually into contact with a sheet from an edge of the sheet and causing a creasing unit to contact the sheet only once; however, this causes a pressure applied onto a center portion of the sheet to be weakened, making it difficult to produce an even crease.
To that end, creasing a sheet gradually from an edge of the sheet to reduce a load during creasing and bringing the creasing unit into contact with the sheet only once for production of an even crease is conceivable. To perform this, it is necessary to retain a sheet to prevent displacement of the sheet during creasing; however, if this sheet retaining operation is performed concurrently with the creasing operation, the sheet is retained only at a portion, which gradually shifts from an edge of the sheet. This can disadvantageously cause displacement of the sheet to occur during creasing.
To that end, a technique of moving a creasing member by using a plurality of individually-advancing-and-retracting mechanisms, which are activated at different times, so as to enable formation of a crease while reducing a pressing force for a creasing member is disclosed in, for instance, Japanese Patent Application Laid-open No. 2009-166928.
A technique of aligning edges of sheets by, when the sheets are cut, pressing a top surface of a batch of the sheets by a first pressing unit, which is capable of ascending and descending to press the batch placed on a sheet stacking unit at a portion near a fold of the batch, by, after a lapse of a predetermined period of time, pressing the batch at a portion near an edge of the batch by a second pressing unit, which is capable of ascending and descending to press the batch, and by, thereafter, trimming the edges of the sheets by a cutting unit, which is capable of ascending and descending is disclosed in Japanese Patent Application Laid-open No. 2000-198613. In this technique, consideration is given to prevention of displacement of the sheet batch.
However, the technique disclosed in Japanese Patent Application Laid-open No. 2009-166928 can disadvantageously cause a crease to have unevenness between a portion of a sheet that comes into contact with a creasing blade multiple times and a portion of the sheet that comes into contact with the creasing blade only once. This technique is also disadvantageous in that it is necessary to retain the sheets at different times by using a plurality of individually-advancing-and-retracting mechanisms to prevent displacement of the sheets during creasing, which also disadvantageously makes the structure complicated.
The technique disclosed in Japanese Patent Application Laid-open No. 2000-198613 prevents displacement of sheets by using the first and the second pressing units to pressing the sheet at different times during sheet cutting; however, the structure according to this technique is complicated as is the structure of the technique disclosed in Japanese Patent Application Laid-open No. 2009-166928. Furthermore, the technique disclosed in Japanese Patent Application Laid-open No. 2000-198613 is for a mechanism that imposes a force of a relatively large magnitude for edge trimming, and not appropriate for a mechanism that performs creasing by placing a relatively light load.